The present invention relates to an improved method and apparatus for ion mobility spectrometry. In particular the invention provides a method and apparatus that yield a higher information content of the obtained ion-mobility spectra and a better probability of correct identification of hazardous substances and a better distinction between hazardous and non-hazardous chemical and biological agents. The method and apparatus of the invention can be used for the analysis of ions of macromolecules for environmental screening, e.g. the detection of proteins and lipids that occur in hazardous biological agents. In particular the improved method and apparatus for ion mobility spectrometry are useful for the detection of biological weapons made from viruses or bacterial spores and inorganic and organic surfactants and other chemicals, e.g. micrometer-sized dust-forming silicate particles.
Ion mobility spectrometry is a powerful analytical tool for the detection of chemical and biological hazards. Typically, in an ion mobility spectrometer (IMS) the sample is ionized, passed through an electric field and the time-of-flight of the different sample ions at atmospheric pressure is detected by an electrode detector. The disadvantage of these prior art IMS is that the false alarm rate for the detection of some chemical and biological hazards is too high for many important civil applications. Some mass spectrometers (MS) have better false alarm rates, but MS are very expensive since they require complicated vacuum technology (see e.g. U.S. Pat. No. 6,342,393). The purpose of this invention lies in an improved method and apparatus for ion mobility spectrometry to obtain a significantly improved accuracy of detection.
In a first embodiment, an IMS is set out wherein molecular ions of the sample are dissociated into fragment ions, and in which the spectra of the fragment ions and the process of fragmentation over time are analyzed. For example, electromagnetic or electron beams may create fragmentation which increases the number of different ions that are detected by the detector of the ion-mobility spectrometer. For detection of biological hazards, the sample may be collected by a virtual impactor, partially chemically decomposed in a pyrolyzer and separated into fractions in gas chromatograph before being analyzed in the IMS. In a further embodiment of the methods and apparatuses of the invention, the interaction of the sample ions with each other over time is monitored and used to achieve a higher information content. Beyond this, in one embodiment of the methods and apparatuses of the invention, a chemical that interacts with the sample is added to the inert gas of the ion mobility spectrometer and the changes of the ion mobility spectra are monitored and used for obtaining a higher information content. This chemical can e.g. be a pH-modifier. Beyond this, in another embodiment of the methods and apparatuses of the invention, larger particles are detected with an ion-mobility spectrometer by using the reversion of the flow of the inert gas relative to the common direction and thereby dragging large particles towards the collector electrode, and using this detection to obtain a higher information content about the sample, e.g. about the presence of weapons-typical additions to spores and viruses. In the embodiments which comprise multiple gatings, before injecting a new sample into the ionization chamber, a higher yield of collected ions may be achieved which may lead to a further improvement of signal/noise ratios. Said ion mobility spectrometers may be operated in the positive or negative ion mode or in both ion modes. The ionization of a target compound of the sample can be done directly by an ionization source that emits energy that interacts with and ionizes the target compound. Alternatively or additionally, a target compound of the sample can be indirectly ionized by an ionization source which emits energy that interacts with and ionizes an intermediate compound which, in turn, interacts with and ionizes the target compound. It should be understood that his invention has been disclosed so that one skilled in the art may appreciate its features and advantages, and that a detailed description of specific components and the spacing and size of the components is not necessary to obtain that understanding. Many of the individual components of the ion mobility spectrometers are conventional in the industry, and accordingly are only schematically depicted. The disclosure and description of the invention and the examples are thus explanatory, and various details in the construction of the equipment are not included. Alternative embodiments and operating techniques will become apparent to those skilled in the art in view of this disclosure, and such modifications should be considered within the scope of the invention, which is defined by the claims. The invention described can of-course also be used in combination with the known prior art variants of ion-mobility spectrometry.